Magnetic vortex switching induced by spin-polarized ac currents of different amplitudes and frequencies is investigated by micromagnetic calculations in Permalloy nanodots of systems lacking inversion symmetry.Micromagnetic simulations reveal that the critical current density for vortex switching can be decreased to 106 A cm-2, mainly due to the current-induced effective transverse field by Rashba effect, and the current with fiequencies close to the vortex eigenfrequency is the most efficient for reversal.